The collection of LNG for analysis is well known. There are standards established for how to sample LNG to provide a representative sample for analysis. Analysis is typically performed in order to establish the BTU value of the LNG to properly price the product. LNG is not a pure compound, but a mixture of various components with each component having a potentially different BTU value. There may also be impurities such as moisture, nitrogen and other components that can affect the BTU value of the LNG. The LNG may be sampled either in a somewhat continuous manner by extracting samples from, for example, a pipeline or loading arm and conveying the samples to the analytical equipment through a piping system. Such a system is often used at a ship offloading site. Another method is to take samples in a “batch” manner using sampling cylinders. Both processes are well known in the art and can be found for example in ISO 8943 or API 17.10 for LNG or API 14.1 and GPA 2166 for typical natural gas. While sampling of natural gas in its gaseous state is relatively straight forward, sampling liquified natural gas (LNG) poses special problems.
Natural gas can be liquified by changing its pressure and/or temperature. At atmospheric pressure, natural gas is a liquid at about −250° F. or colder. At 4000-5000 psi, natural gas can be maintained as a liquid, at ambient temperatures. If the temperature is elevated or pressure is reduced, one or more components of the LNG can vaporize skewing the distribution of the various compounds making up the natural gas in the sample thereby adversely affecting the analysis and its validity in indicating the BTU value of the gas. In an onsite monitoring system where the analytical equipment is connected to the source of LNG, the vaporization of the LNG is intentionally effected at some point prior to the sample reaching the analytical equipment, for example a gas chromatograph. In the process of continuous sampling, i.e., the taking of samples from a source and conveying through a piping system to an analyzer, a probe or other device is inserted into the source, for example, a pipeline or tank. The probe has an open end that exposes the flow path and the piping system to flow communication of LNG from the source. Thus, if the conditions in the piping system are such as to allow the LNG to vaporize, backflow in the piping system can occur and affect the representative nature of the sample within the sampling piping system. If LNG vaporizes, its volumetric expansion from liquid to gas is on the order of 600 fold resulting in an increase in pressure and temperature. This expansion can cause backflow of product in the sampling lines. This then can lead to the providing of a non-representative sample to the analyzer for analysis. Lighter components will take the course of least resistance and can provide for a disproportionate representation of heavier components to enter the analyzer.
It would thus be desirable to provide a continuous sampling system and method that reduces the risk of providing non-representative samples for analysis.